1. Field of the Invention
The present invention relates to an ignition timing control device for electronic control of the ignition timing in an engine.
2. Description of the Prior Art
FIG. 1 is a block diagram showing the arrangement of a prior art ignition timing control device, in which the numeral 1 is a crank shaft of a four-stroke-cycle four-cylinder engine while 2 is a disk mounted to the crank shaft 1 for revolution about the same. The disk 2 has a couple of magnetic members 3A and 3B fixedly mounted on the circumference thereof and located 180 degrees apart.
There are electromagnetic pickups 4C and 4D provided proximate to the periphery of the disk 2 for generation of reference position pulses Pc and Pd respectively when facing each of the magnetic members 3A and 3B. The electromagnetic pickup 4D is arranged at an angle of 90 degrees to the electromagnetic pickup 4C so that they can produce their respective reference position pulses Pc and Pd alternately upon detecting the magnetic members 3A and 3B while the crank shaft 1 rotates 90 degrees. The angular position of the crank shaft detected by the electromagnetic pickup 4D represents a most delayed ignition timing in the subsequent ignition.
An oscillator 5 produces an output of clock pulse CP. With reference to the clock pulse CP from the oscillator 5, a period measuring means 6 measures a period Tc of the reference position pulse Pc.
On the other hand, an ignition timing computing means 9 computes an ignition timing advance .theta. with reference to a reference position of the crank shaft detected by the electromagnetic pickup 4D, according to the information S including the revolutions of the engine and the manifold pressure. Also, an energizing time computing means 20 computes an energizing time Tl, which is required for increasing the primary current of an ignition coil 42 to a specified value, from a battery voltage U.
Then, an ignition preparatory time computing means 10 computes a time Ts, which is taken from the generation of the reference position pulse Pc to the ignition, in a particular manner described later synchronously with the reference position pulse Pc upon receiving the period Tc and the ignition timing advance .theta.. According to the time Ts and the energizing time Tl of the ignition coil 42, an energizing start time computing means 21 computes a time Toff taken for energizing the ignition coil 42 after the generation of the reference position pulse Pc. Upon receiving the clock pulse CP, reference position pulse Pc, and energizing start time Toff, an energizing start signal generating means 22 produces an energizing command signal Pon for commencement of energizing the ignition coil 42 when the energizing start time Toff has passed from the generation of the reference position pulse Pc.
According to the time Ts sent from the ignition time computing means 10, the clock pulse CP, and the reference position pulse Pc, a first ignition signal generating means 30 produces an ignition signal Pspk for stopping the current on the ignition coil 42 when the time Ts has passed from the input of the reference position pulse Pc. Also, a second ignition signal generating means 31 produces a second ignition signal Psd as synchronizing with the input of the reference position pulse Pd.
Then, an ignition control signal generator means 40 converts the output to an ignition means 41 from "L" level to "H" level synchronously with the energizing command signal Pon and also, outputs an ignition control signal Ps which converts the output to the ignition means 41 from "H" level to "L" level in synchronization with whichever of the first and second ignition signals Pspk, Psd comes earlier. Consequently, the ignition means 41 is actuated by the ignition control signal Ps thus to activate the ignition coil 42 for ignition in the engine.
FIG. 2 is a time chart showing the timing of signal outputs from the components of the prior art ignition timing control device. As a series of the reference position pulses Pc sent from the electromagnetic pickup 4C are designated as Pc1, Pc2, and Pc3 (FIG. 2-a), a pulse period Tc2 between Pc1 and Pc2 can be measured by the period measuring means 6 upon input of the reference position pulse Pc2 and with reference to the clock pulse CP from the oscillator 5.
When the reference position pulse Pcs is output, the ignition time computing means 10 computes the time Ts taken for the next ignition (FIG. 2-c), as referring to the pulse period Tc and the ignition timing advance .theta., from: ##EQU1##
An ignition signal Pspk2 for instruction of current cut-off on the ignition coil 42 is then generated by the first ignition signal generating means 30 in the manner described previously, when the resultant time Ts2 determined by the equation (1) has passed from the generation of the reference position pulse Pc2. Accordingly, the energizing start time computing means 21 can compute the time Toff which is taken for energizing the ignition coil 42 after the generation of the reference position pulse Pc2 within a corresponding ignition period to the ignition.
More specifically, the energizing start preparatory time Toff is determined by subtracting the energizing time Tl given by the energizing time computing means 20 from the time Ts given by the ignition timing computing means 10 for ignition after the input of the reference position pulse Pc. Then, the equation is: EQU Toff=Ts-Tl (2)
The energizing command generating means 22 is actuated upon generation of the reference position pulse Pc in order to produce the energizing command signal Pon after the energizing start time Toff has passed (FIG. 2-d). The ignition control signal generating means 40 starts energizing the ignition coil 42 on receiving the energizing command signal Pon. The ignition control signal Ps is then produced (FIG. 2-f) upon generation of whichever comes in first of the first ignition signal Pspk and the second ignition signal Psd (FIG. 2-e) synchronized with the reference position pulse Pd (FIG. 2-b). Accordingly, a current on the ignition coil 42 is cut off and ignition occurs in the engine.
In such an arrangement, the ignition timing control device of FIG. 1 allows the period Tc of the reference position pulse Pc to be uniform during constant revolution in the engine and also, the first ignition signal Pspk to be generated earlier than the second ignition signal Psd. As the result, the ignition is turned on by the first ignition signal Pspk.
FIG. 3 is a time chart showing the timing of signal outputs during acceleration. During the acceleration, the revolution of the engine increases in each action of ignition while the period Tc of the reference position pulse Pc reduces simultaneously. This causes the actual period of the reference position pulse Pc to be shorter than the period Tc used for calculation with the equation (1). As a result, when the ignition lead angle .theta. is approximate to zero, the second ignition signal Psd (FIG. 3-e) will be produced earlier than the first ignition signal Pspk (FIG. 3-c). The ignition is thus turned on by the ignition control signal Ps (FIG. 3-f) which is output synchronously with the second ignition signal Psd, or the reference position pulse Pd. Accordingly, the ignition is prevented from occurring after the reference position pulse Pd during the acceleration.
If ignition occurs without the second ignition signal Psd but with the first ignition signal Pspk, a spark is generated with a great delay of time in the ignition of engine during the acceleration as the ignition signal Pspk is given considerably later than the reference position pulse Pd. This may cause a trouble such as a decline in the output of the engine. It will thus be understood that the ignition control with the second ignition signal Psd is an appropriate method for ensuring a responsive action of ignition during the acceleration in the engine.
However, the prior art ignition timing control device utilizes the second ignition signal Psd to avoid an excessive delay of timing in the ignition during acceleration but the timing of allowing the ignition coil 42 to be energized is still the energizing time Tl earlier than the first ignition signal Pspk (FIG. 3-d). Under such a condition that the ignition is turned on by the second ignition signal Psd which comes earlier than the first ignition signal Pspk, after the commencement of energizing by the energizing command signal Pon, an actual energizing period of time (ps2 of FIG. 3-f) will be shorter than the energizing time Tl.
For example, as the engine is shifted from idling to racing with a radical change in the rotation, the revolutions of the engine increases in number of engine rotation about 100 rpm per one ignition cycle under the engine speed at about 1000 rpm. The energizing time will thus be least in length on the prior art ignition timing control device when the ignition lead angle .theta. is zero. In the above-described racing condition, the time Ts taken until ignition at 1000 rpm (Tc=15 ms) is then represented by Ts=15 (ms) from the equation (1). When the revolutions of engine increases constantly in steps of 100 rpm per an ignition cycle, the time Tcd from the generation of the reference position pulse Pc to the ignition signal Psd will be 14.3 (ms). Thus, a deficiency in the energizing time is 0.7 (ms) calculated from Tcd-Ts.apprxeq.-0.7 (ms).
If the ignition lead angle .theta. is 2.degree., the ignition time Ts is 14.7 (ms). This causes the energizing time to be less 0.3 (ms) in length.
Particularly, for use in an up-to-date engine designed to run at a high speed of revolution, e.g. a DOHC engine, an ignition coil in which the primary current can be built up quickly for having a specified rate of secondary output in a short period of energizing time, is employed to ensure a measure of voltage output at the high speed of revolution. In the ignition coil having the energizing time Tl of 3 (ms), the secondary output voltage will decrease 25% with a small loss of 0.7 (ms) in the energizing time. This may cause misfiring and combustion fault and thus, result in loss of engine power.
Even if the energizing time Tl is precisely computed by the energizing time computing means 20 for having a specified rate of secondary output, it will be shortened during acceleration and lack of energizing time for the ignition coil is inevitable.